A mobile communication system for delivering communications between a transmitter (interrogator) stationarily mounted in a certain fixed place (hereinunder referred to as "on the fixed side") and a simple receiver (transponder) called tag which is mounted on a mobile object (hereinunder referred to as "on the mobile side") has attracted attention. Such a mobile communication system is used for, for example, identifying an object or a person in a communication zone noncontactingly from an identification signal reflected by the transponder with respect to an interrogation signal emitted from the interrogator to the transponder carried by said object or person.
In most of such mobile communication systems, batteries are used as a power source of the transponder due to its simplicity and light weight, and one of the problems to be solved is to prolong the life time of the battery by reducing the power dissipation.
To solve this problem, a power is only supplied to a circuit 64 which is necessary for the reception of an interrogation signal and at the point where the interrogation signal is received, a switching circuit 600 is actuated so as to feed a remaining signal processing means 65, thereby reducing the constant power dissipation, as shown in FIG. 8 (e.g., Japanese Patent Laid-Open No. 201177/1986. Hereinunder, this communication system is referred to as the former communication system).
As shown in FIG. 9, there is another mobile communication system for identifying data by using a compact transponder 70 having a simple structure which receives an unmodulated signal from an interrogator 75 and reflects a response signal imparted with identification data to the interrogator 75 (e.g., Japanese Patent Laid-Open No. 79476/1982. Hereinunder, this communication system is referred to as the latter communication system). However, in the latter communication system shown in FIG. 9 having the above-described structure, only the identification data which is set in advance can be read out. In addition, in order to make the transponder 70 compact and sufficed with a simple circuit structure, a code generator 74 and a low-frequency signal generator 73 for generating the data must also have a simple structure, so that the identification data are limited to several hundred bits (several ten bytes) at most and storage and control of a large amount of information such as several thousand bytes is very difficult. It is necessary for the interrogator 75 to store and control all the induced information based on the identification data read out, and in a system having an increased number of transponders 70 and a plurality of interrogators 75, there are problems to be solved for practical use such as an enlarged and complicated system due to the increase in memories and the necessity for networking. The former communication system has been aimed at only identifying the transponder and reading data from the transponder. In recent years, however, the application of the communication system to a wider field necessitates emitting from the interrogator to the transponder various command signals for commanding the delivery of necessary data or writing various data signals from the interrogator in the memory of the transponder.
In this case, it is necessary to increase the data transmission speed i.e., the rate of transmission of data, in order to communicate necessary command or data signals within a short time in which the transponder passes the communication zone with the interrogator. For this purpose, it is necessary to set the frequency band of the receiver of the transponder at a frequency so high as to sufficiently cover the data transmission speed, which inevitably increases the electric power dissipation of the receiver to which the power is constantly supplied.
To state this in detail, the former communication system is based on the idea that although it is necessary to constantly supply a power to the starting circuit 64 (generally including an amplifier) for receiving a wake-up signal, this system is sufficed with a smaller power dissipation than the system for supplying a power to the signal processing means composed of a frequency converter and a code converter. In this case, in the case of writing a data signal to the transponder, the data signal is subsequently received after the wake-up signal is received. Therefore, in order to transmit a high-speed data signal, i.e., a data signal transmitted at a high data rate, the starting circuit 64 is required to have a high-frequency band corresponding to a data transmitting speed so as to prevent the distortion of the waveform of the high-speed data signal and to adequately obtain a predetermined gain.
Since the current dissipation of such a circuit (a signal amplifier included in the starting circuit) generally increases in proportion to the height of the frequency band, the current dissipation of the starting circuit 64 during the period of waiting for the reception of the high-speed data signal greatly increased in the former communication system. In addition, when the transmission speed is increased and the data transmission time is shortened, the ratio of the power which is constantly dissipated in the starting circuit 64 to the power dissipation of the transponder as a whole is increased. Therefore, high-speed transmission is impossible in the transponder of the former communication system.
Attempt has been made at utilizing the latter mobile communication system for directing production and managing transportation in the production lines of factories. For this purpose, since a comparatively large amount of data such as several ten characters to several thousand characters is transmitted, the communication system is required to transmit data with certainty and at a high speed within a restricted communication time under a noise environment.
Accordingly, the present inventors have invented a mobile communication system and a mobile communication device which is capable of not only reading out an identification signal but also writing and reading a large amount of information of various kinds to and from a receiver (transponder) on the mobile side at a high speed i.e., at a high data rate, and with a high quality, storing and controlling the information by the transponder, and reducing the power dissipation and prolonging the life time of the transponder in spite of high-speed transmission.